I just dont now about the bubbles. If you added the right airbrushing colors like various shades of gray, blue, silver and white I bet they would be very believable over BC piped dots.
The water idea using the piping gel is a great one. I also understand you can make a sugar syrup tinted with blue and pour it on but it is really a tough trick to master. I have not tried either one as yet but I so want to see ho yours turns out! My curiosity is peaked!
Hi I made the bathtub cake for a baby shower. I used large and small gumballs and stacked them, and "glued" them with white icing. Then they could be eaten, and looked great since they are shiney and the right size, good luck, I can email you a pic of mine if you are interested let me know firstname.lastname@example.org